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    SubjectRe: [PATCH] writeback: fix writeback cache thrashing
    On Sat, Jan 05, 2013 at 03:41:54AM -0600, Simon Jeons wrote:
    > On Sat, 2013-01-05 at 15:38 +0800, Fengguang Wu wrote:
    > > On Fri, Jan 04, 2013 at 11:26:43PM -0600, Simon Jeons wrote:
    > > > On Sat, 2013-01-05 at 11:26 +0800, Fengguang Wu wrote:
    > > > > > > > Hi Namjae,
    > > > > > > >
    > > > > > > > Why use bdi_stat_error here? What's the meaning of its comment "maximal
    > > > > > > > error of a stat counter"?
    > > > > > > Hi Simon,
    > > > > > >
    > > > > > > As you know bdi stats (BDI_RECLAIMABLE, BDI_WRITEBACK …) are kept in
    > > > > > > percpu counters.
    > > > > > > When these percpu counters are incremented/decremented simultaneously
    > > > > > > on multiple CPUs by small amount (individual cpu counter less than
    > > > > > > threshold BDI_STAT_BATCH),
    > > > > > > it is possible that we get approximate value (not exact value) of
    > > > > > > these percpu counters.
    > > > > > > In order, to handle these percpu counter error we have used
    > > > > > > bdi_stat_error. bdi_stat_error is the maximum error which can happen
    > > > > > > in percpu bdi stats accounting.
    > > > > > >
    > > > > > > bdi_stat(bdi, BDI_RECLAIMABLE);
    > > > > > > -> This will give approximate value of BDI_RECLAIMABLE by reading
    > > > > > > previous value of percpu count.
    > > > > > >
    > > > > > > bdi_stat_sum(bdi, BDI_RECLAIMABLE);
    > > > > > > ->This will give exact value of BDI_RECLAIMABLE. It will take lock
    > > > > > > and add current percpu count of individual CPUs.
    > > > > > > It is not recommended to use it frequently as it is expensive. We
    > > > > > > can better use “bdi_stat” and work with approx value of bdi stats.
    > > > > > >
    > > > > >
    > > > > > Hi Namjae, thanks for your clarify.
    > > > > >
    > > > > > But why compare error stat count to bdi_bground_thresh? What's the
    > > > >
    > > > > It's not comparing bdi_stat_error to bdi_bground_thresh, but rather,
    > > > > in concept, comparing bdi_stat (with error bound adjustments) to
    > > > > bdi_bground_thresh.
    > > > >
    > > > > > relationship between them? I also see bdi_stat_error compare to
    > > > > > bdi_thresh/bdi_dirty in function balance_dirty_pages.
    > > > >
    > > >
    > > > Hi Fengguang,
    > > >
    > > > > Here, it's trying to use bdi_stat_sum(), the accurate (however more
    > > > > costly) version of bdi_stat(), if the error would possibly be large:
    > > >
    > > > Why error is large use bdi_stat_sum and error is few use bdi_stat?
    > >
    >
    > Thanks for your response Fengguang! :)

    You are welcome.

    > > It's the opposite. Please check this per-cpu counter routine to get an idea:
    > >
    > > /*
    > > * Add up all the per-cpu counts, return the result. This is a more accurate
    > > * but much slower version of percpu_counter_read_positive()
    > > */
    > > s64 __percpu_counter_sum(struct percpu_counter *fbc)
    > >
    > > > >
    > > > > if (bdi_thresh < 2 * bdi_stat_error(bdi)) {
    > > > > bdi_reclaimable = bdi_stat_sum(bdi, BDI_RECLAIMABLE);
    > > > > //...
    > > > > } else {
    > > > > bdi_reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
    > > > > //...
    > > > > }
    > > > >
    >
    > The comment above these codes:
    >
    > * In order to avoid the stacked BDI deadlock we need
    > * to ensure we accurately count the 'dirty' pages when
    > * the threshold is low.
    >
    > Why your meaning threshold low is error large?

    Because bdi_reclaimable is normally less than or at least comparable
    to bdi_thresh. So (bdi_thresh < 2 * bdi_stat_error(bdi)) means the
    resulted bdi_reclaimable will be small, so small that the bdi_stat()
    error is not ignorable and should be avoided.

    Thanks,
    Fengguang

    >
    > > > > Here the comment should have explained it well:
    > > > >
    > > > > * In theory 1 page is enough to keep the comsumer-producer
    > > > > * pipe going: the flusher cleans 1 page => the task dirties 1
    > > > > * more page. However bdi_dirty has accounting errors. So use
    > > >
    > > > Why bdi_dirty has accounting errors?
    > >
    > > Because it typically uses bdi_stat() to get the rough sum of the per-cpu
    > > counters.
    > >
    > > Thanks,
    > > Fengguang
    > >
    > > > > * the larger and more IO friendly bdi_stat_error.
    > > > > */
    > > > > if (bdi_dirty <= bdi_stat_error(bdi))
    > > > > break;
    > > > >
    > > > >
    > > > > Thanks,
    > > > > Fengguang
    > > >
    >
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